T cells spend the majority of their time perusing lymphoid organs in search of cognate antigen presented by antigen presenting cells (APCs) and quickly recirculating through the bloodstream to another lymph node. Extravasation into a given lymph node occurs through high endothelial venules (HEV) and is dependent upon several key events, including activation of chemokine receptor CCR7 by chemokine ligands CCL19/21 and adhesion of integrin LFA-1 (CD11a/CD18) to its ligand ICAM-1. T cells must become highly migratory to scan the densely packed organ for their cognate antigen on APC, utilizing active LFA-1 at the leading edge of a migrating T cell. Upon antigen challenge, a T cell forms a stable interaction with the APC, which is known as the immunological synapse (IS). Active LFA-1 is relocated to the IS, stabilizing the interaction between the T cell and APC. Although LFA-1 is a key adhesive force for both migration and IS formation, the outcome of chemokine and T cell receptor (TCR) signaling is quite different, as the former induce go and the latter mediate stop signals. We hypothesize that the magnitude of chemokine and TCR signals received by the T cell will determine the outcome of LFA-1 mediated interactions and thus T cell activation. To investigate the role of LFA-1 during T cell activation and migration, we generated fluorescent knock-in (KI) mice that will allow us to visualize expression, distribution, and activation pattern of LFA-1. Using OTI TCR transgenic mice together with an altered peptide ligand (APL) system, we will measure LFA-1 redistribution and activation in T cells activated by varying TCR signal strength. This will allow us to determine the effect of TCR signal strength on LFA-1 redistribution in migrating T cells directed to undergo IS formation (Aim 1). We will also incorporate a photoactivatable chemokine receptor, PA-CCR7, to assess the effect of chemokine signal strength on LFA-1 activation (Aim 2). By combining these techniques, we will test whether the balance between chemokine-dependent versus TCR-dependent signals dictates LFA-1 distribution and function, as well as the mechanism balancing the receptor competition to ensure accurate LFA-1 utilization in a migrating T cell versus an activating T cell engaged in an immunological synapse.